Page 392 - Fundamentals of Radar Signal Processing

FIGURE 5.25 The use of two PRFs to avoid blind speeds in pulse Doppler
radar. The target Doppler shift equals the PRF in the upper sketch, but not in the
lower sketch.

In some systems, as many as eight PRFs may be used. The first velocity that
is blind at all of the PRFs is the LCM of the individual blind speeds, which will
usually be much higher than any one of them alone. Target detections are
typically accepted and passed to subsequent processing only if they occur in
some minimum fraction of the PRFs used, for example two out of two or three

out of eight PRFs.
The advantages of a CPI-to-CPI stagger system are that range-aliased
clutter can be canceled within each CPI using coherent MTI and that the radar
system stability, particularly in the transmitter, is not as critical as with a pulse-
to-pulse stagger system (Schleher, 1991). The disadvantages are that the overall
velocity response may not be very good and that the transmission of multiple
CPIs consumes large amounts of the radar timeline and energy.

This discussion shows that for a given PRF there are periodic blind zones
in Doppler spaced by the PRF. A target whose Doppler shift is such that it
either is in the clutter region or aliases to the clutter region is unlikely to be
detected. Equivalently, if the target falls into a clutter region caused by the
periodically repeated clutter spectrum will likely go undetected. The idea is

illustrated in Fig. 5.26a, which indicates clutter at zero Doppler and its first two
periodic repetitions at ±PRF Hz. The target shown just above PRF Hz but still
within the clutter region is considered to be in the blind zone centered at F =
D
PRF Hz.